Juvenile salmon body condition in Southeast Alaska is buffered during marine heatwaves Marine Ecology Progress Series

Pacific salmon (Oncorhynchus spp.) are ecologically, culturally, and commercially valuable throughout the Pacific Rim. Early marine survival of juvenile salmon is crucial for maintaining population abundance and sustainable fisheries. Climate variability, including multiple marine heatwaves, have recently caused ecosystem-wide changes and stressors with reduced return rates for some salmon populations in the Gulf of Alaska. We examined dietary niche breadth and condition metrics in juveniles of 4 salmon species (chum O. keta, coho O. kisutch, pink O. gorbuscha, and sockeye O. nerka), spanning 2010 to 2019, including 2 prominent heatwaves (2015-2016 and 2019). Samples were collected annually in Icy Strait, a major fish migration corridor in the Southeast Alaska archipelago. Analyses included bulk δ15N and δ13C stable isotopes, stomach contents, energy density, and morphometrics to evaluate variability in niche breadth and condition in response to extreme climate events. Interannual variability in stable isotopes was primarily explained by sea surface temperature (SST) anomalies for all species. The inverse relationship between SST and δ13C suggests the potential for increased terrestrial input into the nearshore environment during warmer conditions. We also found weak relationships between interannual variability in stomach contents and SST anomalies for pink salmon and a combination of δ13C, δ15N, SST, and condition for coho. However, fish condition did not show a significant relationship with SST anomalies. We propose that the complex topography and oceanography of northern Southeast Alaska provide marine habitat that temporarily buffers juvenile salmon from declines in body condition during heatwaves prior to migration into the Gulf of Alaska.